1. Field of The Invention
The invention relates to synchronous (synonymous with "coherent") communication receivers demodulating digital information from an amplitude-, phase- and/or frequency-shift keyed signal. In particular, the invention relates to such receivers which must first determine the carrier frequency of the signal to be received (this is called "frequency acquisition"), from within a frequency band which is much wider than the signal spectrum.
2. Description of The Prior Art
The frequency acquisition process is frequently unacceptably slow. When the signal is weak, but still perfectly useable, it is possible that the wideband noise power will be significant in comparison to or even greater than the signal power. Finally, coherent receivers for frequency-shift keying or continuous-phase frequency-shift keying usually require joint acquisition of clock and carrier, which in turn demands very slow frequency sweeping once coarse frequency acquisition has been made. The acquisition time may be greater than several seconds, which is longer than users of microwave radios are accustomed to. In addition, this slow frequency sweeping invites false receiver locks onto data sidebands.
This field of receiver design and its associated problems have recently been commercially realized with the new low-cost point-to-point digital radio systems for telecommunications. These systems employ lower data rates and higher carrier frequencies than have previously been used in the industry. The higher carrier frequencies and economical transmitter designs lead to larger carrier frequency drifts. At the same time, the lower data rates require correspondingly narrower signal spectra to obtain good communication efficiency. Good communication efficiency is a prime requirement at these higher carrier frequencies where transmitter power is expensive and atmospheric attenuation necessitates large fade margins. The result is that the carrier frequency drift is now the predominant contributor to the IF bandwidth requirement, being much larger than the signal spectrum. For example, consider a typical radio system communicating 1.544 Mb/sec on a minimum-shift-keyed carrier of 23 GHz, with the transmitter and receiver first local oscillator each contributing a frequency tolerance of 0.03%. The frequency drift is 2 times 0.03% of 23 GHz, which is 13.8 MHz. Since this may be in either direction, the IF bandwidth required to contain the frequency drift is twice this, or 27.6 MHz. At the same time, 99% of the signal power is contained in a bandwidth 1.2 times 1.544 MHz which is 1.85 MHz, very small by comparison. False receiver locks will occur at three or more frequencies on each side of the actual carrier, spaced in multiples of 772 KHz from the carrier.
There are, however, three characteristics of the commercial single-channel-per-carrier digital radio environment which may be exploited. First, receivers require 8 decibels or more of energy-per-bit to noise-density ratio (Eb/No). Second, they require ten or more decibels of signal-to-intereference power ratio (C/I). These two constraints on system design insure that the signal power spectral density will exceed the noise power spectral density by eight decibels or so in a narrow band centered on the carrier frequency, and that interfering signals in the frequency band to be searched will be ten decibels or so less in power than the desired signal. Third, receivers may be designed with signal-acquired indicators which are immune to false locks.
Until recently, commercial digital radio systems have had carrier frequency drifts very small in comparison with their signal spectrum; hence the intermediate frequency (IF) bandwidth has not needed to be much larger than the signal spectrum. In this regime, the signal-to-noise ratio at the output of the IF amplifier is at least ten decibels or so and the conventional frequency discriminator has been very succesfully used to aid in coarse-tuning the frequency in preparation for phase-locking by the local carrier oscillator.
Systems such as the 23 GHz radio described above have sometimes used the frequency discriminator in spite of its inherent difficulties, with the frequent failure to acquire weak signals in a very sensitive receiver. In addition, in synchronous systems the problem of escaping the false locks, which surround the true lock in a coherent receiver, does not seem to have been satisfactorily solved.